it appeals to that primitive instinct of belonging to a pack, fostering an us-vs-them mentality. this way, you are less likely to leave and are encouraged to give your 110%

It's also not just a tech thing, in consulting you have your McKinseyites, BCGers and Bainies

A shortcut to avoid calling them "Palantir employees"?!

I think in this situation, it's the less nefarious explanation that probably reflects reality.

It's probably just a shortcut for "X-company employees"

Is it an American thing? We name folks from counties and states with nicknames too - Hoosiers from Indiana for instance. And folks from Cedar Rapids are Cedar-Rapidians around here.

It's not just an American thing, it's the concept of a "demonym" -- and there are official and unofficial demonyms. For example, the official demonym of Massachusetts is "Bay Stater," but everyone uses the far more popular "masshole."

Internal reports are always written in Clingon!